Monitoring system for push-pull amplifiers



: Oct. 30, 1934- J. H. HAMMOND, JR

MONITORING SYSTEM FOR PUSH-PULL AMPLIFIERS Filed Nov. 23, 1929 3Sheets-Sheet l PIC-5.1;

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9 JOHN HAYS HAMMOND m Oct. 30, 1934.

J. H. HAMMOND, JR

MONITORING SYSTEM FOR PUSH-PULL AMPLIFIERS Filed Nov. 25, 1929 FIGA.

3 Sheets-Sheet 2 SIGNAL STRENGTH ouw we flwmx JOH N HAYS HAMMOND JR Oct.30, 1934. v ,1. H. HAMMOND, JR 1,979,036-

MONITORINGSYSTEM FOR PUSH-PULL AMPLIFIERS Filed NOV. 23, 1929 3Sheets-Sheet 5 FIG .5.

SIGNAL STRENGTH OLLVX! NW9 Fm/Mm I, JOHN HAYS HAMMOND JR.

.110 cum Patented Oct. 30, 1934 UNITED STATES PATENT OFFICE John HaysHammond, Jr., Gloucester, Mass., as-

signor to Hammond Holding Corporation, Gloucester, Mass., a corporationof Delaware Application November 23, 1929, Serial No. 409,211

22 Claims.

This invention relates to an automatic control system, and moreparticularly to a system for automatically controlling the amplificationof an electron discharge amplifier in accordance with the'amplitude ofthe signal being translated.

This invention is particularly applicable to pick-up devices, such as aphonograph pick-up or a photo-electric pick-up, and to an amplifyingsystem for amplifying the signals picked up and applying them to a soundpropagating device such as a loud speaker.

The volume ratio between the loud and soft tones of an orchestralselection is considerably greater than that which may be recorded on aphonograph record or photographic sound record. The present invention,therefore, provides for-expanding the volume ratio of the recorded soundand causing the same to more nearly approximate the originalselection.The invention also provides a limiting device for preventing theamplifier from becoming overloaded, with consequent sound distortion,and the amplifier itself is especially constructedto prevent distortiondue to undesired modulation.

The invention particularly provides for the elimination of extraneousnoises, such as needle scratch or film noises, and prevents theamplifier from operating in response to such noises when they areunaccompanied by desired musical tones.

The invention also consists in certain new and original features ofconstruction and combinations of parts hereinafter set forth andclaimed.

Although the novel features which are believed to be characteristic ofthis invention will be particularly pointed out in the claims appendedhereto, the invention itself, as to its objects and advantages, the modeof its operation and the manner of its organization may be betterunderstood by referring to the ollowing description taken in connectionwith the accompanying drawings forming a part thereof, in which Fig. lis a schematic diagram of a monitoring system constructed in accordancewith the present invention;

Fig. 2 is a schematic diagram having a modifled form of control;

Fig. 3 is a schematic diagram of a further modified form of control;

50 Fig. 4 is a diagram of the amplification characteristics of anamplifier 'having the control shown in Fig. 2; and

Fig. 5 is a diagram of the amplification characteristics of an amplifierhaving the control shown in Fig. 3.

Like reference characters denote like parts in the several figures ofthe drawings.

In the following description and in the claims parts will be identifiedby specific names for convenience, but they are intended to be as ge- 50neric in their application to similer parts as the art will permit.

Referring to the drawings more in detail, an amplifying system is shownin Fig. 1 as applied to phonograph pick-up 10 or a photo-electricpick-up comprising photo-electric cell '11 to which the amplifier may beselectively connected by double pole double throw switch 12.

The photo-electric pick-up includes a source of light 14, lens 15,aperture plate 16, having an aperture 17 therein, and photographic soundrecord or film 18, the above elements being arranged so that light raysfrom source 14 are focused by means of lens 15 through aperture 1'7 ontothe photographic sound record and are then used to actuatephoto-electric cell 11. Said cell is energized by a suitable source ofpotential 20 and is connected through transformer 21 with switch 12.

The amplifier is disclosed as comprising three stages of push-pullamplification including elec-- tron discharge tubes 25, 26, 27, 28, 29and 30 and coupling transformers 31, 32, 33 and 34 to which said tubesare connected in the manner well known in the art. The secondary oftransformer 34 is connected to the armature coil 35 of a loud speakingdevice 36 which may have a stationa y field coil 37 energized by battery38. Tubes 25 to 30 have the usual sources of potential for theirfilament, grid and plate circuits, which are well known in the art andwill not be described in detail.

A variable resistance 40 is connected across the input terminals oftransformer 31 to permit the voltage applied to the system to bemanually adjusted as desired. It is obvious, however, that the energyderived from the various pick-up devices may be amplified as desiredbefore being applied to the amplifying system disclosed herein and themanual voltage control may be associated with any convenient part of theapparatus.

The automatic control device is shown as applied to the grid circuit oftubes 2'7 and 28 which comprise the second stage of amplification. Theimpedance of tubes 27 and 28 is controlled by the normal grid potentialthereof. This is deter- Y mined by the potential of battery 45, thesetting manually adjusted to cause tubes 27 and. 28 to pass the desiredamount of plate current as indicated by meter 48.

The current through resistance 47 which determines the potential drop ofsaid resistance is controlled by the volume of the input signal appliedto tubes 27 and 28. For this purpose an impedance chain, comprisingcondensers 50 and 51 and resistances 52 and 53, is connected across thetwo grids of tubes 27 and 28.

The constants of this impedance chain may be chosen by making condensers50 and 51 sufficiently small to substantially block low frequency tones.

Resistance 52 is connected in the input circuit of space dischargeamplifier 55, the output circuit of which is connected throughtransformer 56 to a pair of rectifiers 57 which are connected acrossresistance 47. A condenser 58 may be connected across the output circuitof an amplifier to bypass the high frequency tones and prevent therectifiers from being responsive thereto.

In order to prevent the current in resistance 47 from varying with theinstantaneous value of the signal current a low-pass filter net-work 59may be connected in the output circuit of rectifiers 57 and a condenser60 may be connected across resistance 47.

In the operation of the above described control system rectifiers 57 areoperatedin accordance with the value of the intermediate frequencysignals present in the amplifier circuit. The rectified current ispassed through resistance 47 and causes a potential drop in thatresistance which is proportional to the current flowing therein. Thispotential drop is in such direction that an increased signal in theamplifier circuit decreases the normal potential or bias of the grids oftubes 2'7 and 28, thereby lowering the impedance of said tubes andcausing increased current to flow in the output circuit thereof.Condenser 60 serves to iron out variations in voltage across resistance47 causing the control system to vary the gain ratio according to theaverage value of the signal.

Condensers 50 and 51 prevent the rectifiers from being operated by thelow frequency signals and thereby prevent the control system fromoperating in response to sharp percussive sounds, such as a kettle drum,The higher frequency signals, such as those produced by needle scratchor film noises, are by-passed by condenser 58 and thereby prevented fromoperating the control system.

i Tubes 27 and 28 may be normally adjusted to pass a minimum amount ofcurrent when no signalis being applied. The amplificationcharacteristics will then be determined by the change in grid bias dueto current flowing in resistance 47 when a signal is applied to theamplifier. It is to be noted, therefore, that high frequency noises,such as needle scratch and film noises, are not amplified except whenaccompanied by other signals since rectifiers 57 are not responsive tothose frequencies.

For preventing a loud signal from overloading the amplifier andtherebycausing distortion, an

automatic limiting device may be employed which is operated by theoutput signal. This limiting device comprises a pair of rectifiers 61-which are connected through transformer 62, filter 63 and resistances 64with the output circuit of transformer 34. Rectifiers 61 are connectedthrough a filter network 65 with resistance 66 battery 67 and by thepotential drop in resistance rectifiers cause acurrent to pass throughresistance 66 in such a direction that the grid bias of tubes 25 and 26is increased, whereby their amplification characteristics are lowered.

The overload control may be applied to the input circuit of other stagesof amplification if desired. It is preferably applied to the inputcircuit of the first stage, as shown, since the signal amplitude thereinis smaller and a smaller potential drop may be used to effect thedesired control. Furthermore, the effect of the limiter is increased dueto the action of I the succeeding stages of amplification and the abovedescribed control system.

Distortion in the output circuit due to rectifiers 61 is reduced to aminimum by the use of balanced rectifiers, as shown. The desiredfrequency characteristics may be impartedv to the -in Fig. 2 in whichamplifying tube 55 is controlled by separate secondary winding 75 oftransformer 32. The frequency characteristics of the control circuit aredetermined by band-pass filter 76 which is preferably designed to passfrequencies in the'middle tone range so that control will not be undulyaffected by low or high tones.

The output circuit of transformer 56 in the system of Fig. 2 isconnected through filter 76 to rectifiers 77 which supply direct currentthrough resistance 47 in the manner previously pointed out.

The elements of the circuit identical with those shown in Fig. 1 aregiven similar reference numerals and will not be again described.

A further modified form of control circuit is shown in Fig.- 3- in whichthe amplifier 55 is coupled by transformer 56 to a filter net-work 80which determines the frequency characteristics of the circuit. Theoutput of filter 80 is applied through amplifier 81 and transformer 82to rectifiers 77 which are similar to those illus- 1 5 trated in Fig. 2.A limiting device comprising rectifier 83 and biasing battery 84 isconnected across the input of rectifier 77 for limiting the current inresistance 47 so that on high signal strengths the bias on the inputcircuit of tubes 27 and 28 will not become sufliciently small to causesaid tubes to operate in an abnormal manner.

It is to be noted that by placing the filter system in the controlcircuit instead of in the input to the amplifier, the system is capableof distinguishing between currents caused by needle scratch and desiredcurrents caused by overtones or high notes w ich are present in theamplifier. These high frequencies are not discriminated thereof preventsdistortion due to undesired modulation which might otherwise occur inthe amplifier tubes and insures a practically undistorted signal in theoutput circuit.

While certain novel features of the invention have been shown anddescribed and are pointed out in the annexed claims, it will beunderstood that various omissions, substitutes and changes in the formsand details of the device illustrated and in its'operation may be madeby those skilled in the art without departing from the spirit of theinvention.

What is claimed is:

1. In a sound reproducing system, a sound record, a pick-up devicetherefor, a plurality of stages of amplification fed by sa'id pick-updevice, and means fed by said pick-up device controlling the gain ratiosof said stages in accordance with the strength of the signal derivedfrom said record.

2. In a sound reproducing system, a sound record, a pick-up devicetherefor, a plurality of stages of amplification fed by said pick-updevice, separate control circuits fed by said pickup device controllingthe gain ratios of said stages in accordance with the strength of thesignal.

3. In a sound reproducing system, a sound record, a pick-up devicetherefor, at least three stages of amplification fed by said pick-updevice, each stage comprising a push-pull amplifier, a normal dynamiccontrol circuit fed by the input of an intermediate of said stages, saidcircuit having means for varying the gain ratio of the push-pullamplifier of that stage in accordance with the strength of the signalapplied to the input thereof, a limiting dynamic control circuit fed bythe output of the last stage and having means for varying the gain ratioof the first stage inversely as said normal control circuit, said normalcontrol circuit operating to increase the gain ratio with increase insignal strength, said limiting control circiut operating to decrease thegain ratio with increase in signal strength.

4. In a sound reproducing system, a' sound record, a pick-up devicetherefor, at least three stages'of amplification fed by said pick-updevice, a normal dynamic control circuit fed by the input of 'anintermediate of said stages, said circuit having means for varying thegain ratio of the amplifier of that stage in accordance with thestrength ofthe signal applied to the input thereof, a limiting dynamiccontrol circuit fed by the output of the last stage and having means forvarying thegainratio of the first stage inversely as said normal controlcircuit.

5. In a sound reproducing system, a sound record, a pick-up devicetherefor, an amplifier fed by said pick-up device, a control circuit fedby said pick-up-device having means for rectifying v and averaging thesignal impressed thereon, said control circuit including a full waverectifier and afilter to eliminate the eifect of residual tones on saidamplifier, said control circuit also having means for preventingundesired tones from being rectified and averaged.

6. In a sound reproducing system, a sound record, a pick-up devicetherefor, a push-pull amplifier fed by said pick-up device, a controlcircuit fed by said pick-up device controlling the gain ratio of saidamplifier in accordance with the strength of the signal impressedthereon, whereby the effect of noise in said control circuit isalleviated.

7. In a sound reproducing system, a sound record, a pick-up devicetherefor, an amplifier fed by said pick-up device, a control circuitalso fed by said pick-up device for varying the gain ratio of saidamplifier in accordance with the strength of the signal impressed uponsaid control circuit, said control circuit having a band filter forpreventing the passage therethrough of low frequency tones and highfrequency tones and consequent control of gain ratio thereby.

8. In a sound reproducing system, a sound record, a pick-up devicetherefor, an amplifer fed by said pickup device, a control circuit alsofed by said pick-up device for varying the gain ratio of said amplifierin accordance with the strength 100 of the signal impressed upon saidcontrol circuit, said control circuit having means for preventingcontrol by undesired tone frequencies while permitting control bydesired tone frequencies, whereby the amplification of said undesiredtone frequencies is not affected unless accompanied by desired tonefrequencies.

9. In a sound reproducing system, a sound record, a pick-up devicetherefor, an amplifier fed by said pick-up device, a control circuit fedby said 110 pick-up device having means for rectifying and averaging thesignal impressed thereon, said control circuit including a full waverectifier and a filter to eliminate the effect of residual tones on saidamplifier.

10. In a sound reproducing system, a sound record, a pick-up devicetherefor, an amplifier fed by said pick-up device, a control circuit fedby said pick-up device having means for rectifying and averaging thesignal impressed thereon, said 120 control circuit including a full waverectifier to alleviate the effect of residual tones on said 11. In asound reproducing system, a sound record, a pick-up device therefor, anamplifier fed by said pickup device, a control circuit also fed by saidpick-up device for varying the gain ratio of said amplifier inaccordance with the strength of signal impressed upon said controlcircuit, said control circuit having means for discriminating againstlow frequency signal tones so as to prevent these tones from controllingthe amplification to an undesired extent.

1 In a sound reproducing system, a sound record, a pick-up devicetherefor, an amplifier fed by said pick-up device, a control circuitalso fed by said pick-up device for varying the gain ratio of saidamplifier in accordance with the strength of signal impressed upon saidcontrol element, said control element having means for discriminatingagainst high frequency signal tones so as to prevent thesetones fromcontrolling the amplification to an undesired extent.

13. In a sound reproducing system, a sound record, a pick-up devicetherefor, a dynamic expander fed by said pick-up device, a soundpropagating device fed by said dynamic expander, an output limitingarrangement fed by the output of said dynamic expander for determiningthe maximum amount of dynamic expansion, said limiting 150 dillarrangement having a critical point corresponding to an output belowwhich limiting control is not exercised.

14. In a sound reproducing system, a sound record, a pick-up devicetherefor, a dynamic amplifier fed by said pick-up device, a soundpropagating device fed by said dynamic amplifier, a limiting circuit fedby the output of said dynamic amplifier for controlling the expansioncaused by said dynamic amplifier, said limiting circuit having acritical point below which limiting control is not exercised.

15. In a sound reproducing system, a sound record, a pick-up devicetherefor, a dynamic expander fed by said pick-up device, a soundpropagating device fed by said dynamic expander, a control arrangementfed by said pick-up device for controlling the passage of energy throughsaid dynamic expander, said control arrangement having a limiting devicefor limiting the amount of dynamic expansion.

' 16. In a sound reproducing system, a sound record, a pick-up devicetherefor, a dynamic amplifier fed by said pick-up device, a soundpropagating device fed by said dynamic amplifier, a control circuit fedby said pick-up device for controlling the gain ratio of said amplifierin accordance with the average intensity of signal, said control circuithaving a limiting device for limiting the maximum control energy.

-17. In a sound reproducing system, a sound record, a pick-up devicetherefor, a push-pull amplifier fed by said pick-up device, a soundpropagating device fed by said amplifier and a control circuit forcontrolling the gain ratio of said amplifier according to a movingaverage of the signal intensity.

18. In a sound reproducing system, a sound record, a pick-up devicetherefor, a plurality of stages of push-pull amplification fed by saidpickup device, the gain ratios of said stages being controlled inaccordance with the strength of the signal derived from said record.

19. In a sound reproducing system, a sound record, a pickup devicetherefor, an amplifier fed by said pick-up device, a control circuitalso fed by said pick-up device, said control circuit having a rectifiersupplying a sub-audio current for controlling the gain ratio of saidamplifier in accordance with the strength of signal impressed on saidcontrol circuit, and means for preventing high frequency tones fromreaching said rectifier, whereby dynamic control of said amplifier bysaid high frequency tones is prevented.

20. In a sound reproducing system, a sound record, a pick-up devicetherefor, a sound propagating device fed by said pick-up device, saidrecord having audio tones of 1ow,intermediate and high frequencies, andmeans controlled exclusively by the presence of the intermediatefrequencies in the signal'picked up from the record for determining thedynamic expansion of tones of all frequencies.

21. In a sound reproducing system, a sound record, a pick-up devicetherefor, a sound propagating device fed by said pick-up device, saidrecord having audio tones of low, intermediate and high frequencies, andmeans controlled exclusively by the presence of the intermediatefrequencies in the signal picked up from the record for determining theintensity of tones of all frequencies.

22. In a sound reproducing system, a sound record, a pick-up devicetherefor, a sound propagating device, a plurality of stages of push-pullamplifiers between said devices and a dynamic volume control associatedwith an intermediate one of said amplifier stages.

JOHN HAYS HAMMOND, JR.

